1. Field of the Invention
The present invention relates to a solenoid valve, and a modulator and an anti-lock brake system with the solenoid valve, more particularly to a 2-position 3-way (hereinafter simply referred to as "2/3") solenoid valve for an anti-lock brake system, a compact type of modulator and an anti-lock brake system with the solenoid valve.
2. Description of the Prior Art
Presently, an anti-lock brake system which prevents wheels from locking while performing a braking operation of a vehicle is widely utilized. If wheels are locked during the braking operation to slip on a road surface, a friction force between tires and the road surface is reduced to lengthen the braking distance. The anti-lock brake system repeatedly increases, holds or decreases the braking pressure exerted on the wheels to impede the locking of wheels. The increase of braking pressure is generally referred to as an increase mode, the hold thereof as a hold mode, and the decrease thereof as a decrease mode. The anti-lock brake system is generally composed of a pressure generating source, which is not a master cylinder, for example, a pump and the like, valves operated by an electrical signal, sensors for monitoring a rotating speed of the wheels and a controller for opening/closing the valves in accordance with a predetermined algorithm for the purpose of acquiring an effective braking operation.
FIG. 1A is a schematic hydraulic circuit diagram for showing a conventional anti-lock brake system utilizing 2-position 2-way (hereinafter simply referred to as "2/2") solenoid valves. In an increase mode, first and second 2/2 solenoid valves S220 and S 221 are de-energized to open first valve S220 connected to an outlet of a hydraulic pump 120 and close second valve S221 connected to an inlet of hydraulic pump 120, so that a hydraulic pressure generated from hydraulic pump 120 is supplied to brake wheel cylinder mounted to the wheels. In a hold mode, first valve S220 is energized to close first valve S220, so that the pressure of the brake wheel cylinder is maintained constant. In a decrease mode, first and second valves S220 and S221 are energized to close first valve S220 and open second valve S221, so that the pressure of the brake wheel cylinder is decreased. Such a system is stably operated, but two solenoid valves per channel are required. As a result, the utilized number of solenoid valves is increased and the overall anti-lock brake system becomes bulky.
FIG. 1B is a schematic hydraulic circuit diagram for showing a conventional anti-lock brake system utilizing 3-position 3-way (hereinafter simply referred to as "3/3") solenoid valves. In an increase mode, a 3/3 solenoid valve S330 is de-energized, so that a hydraulic pressure generated from a hydraulic pump 120 is supplied to the brake wheel cylinder to increase the braking pressure. In a hold mode, a primary switching signal is applied to 3/3 solenoid valve S330 to close all three ports thereof, so that the pressure of the brake wheel cylinder is maintained constant. In a decrease mode, a secondary switching signal, of which the voltage is about twice that of the primary switching signal, is applied to decrease the pressure of the brake wheel cylinder. In such a system, the utilized number of solenoid valves is decreased, since a solenoid valve per channel is utilized. However, the structure of solenoid valve is complicated, the size thereof is large, an electronic control unit of a complicated structure is required because of performing 2-step voltage control and the response time becomes long. As a result, the system is operated unstably.
In order to overcome the foregoing problems, an anti-lock brake system has been proposed as disclosed in U.S. Pat. No. 4,865,399 to Atkins et al. The system is composed of a hydraulic pump, 2/3 solenoid valves allotted to each wheel and a control means. However, the system doesn't overcome the foregoing problems completely, since the number of valves utilized is not decreased enough, i.e., the system further includes solenoid-operated isolation valves as well as the 2/3 solenoid valves. Therefore, the overall construction of the system is complicated by utilizing a plurality of solenoid-operated valves. Further, the construction of the 2/3 solenoid valve utilized in the system is complex, whereby it is not easy to fabricate the valve.
Meanwhile, a 2/3 solenoid valve has been proposed as disclosed in U.S. Pat. No. 5,135,027 to Nobuaki Miki et al. However, the valve is not for an anti-lock brake system but for a general-purpose hydraulic system. The valve has the construction that a port is closed by the movement of a ball caused by the flow of brake fluid, and hence, the ball moves by the pressure difference. In other words, the ball moves not by the movement of a plunger but by the pressure difference between ports, though electric power is applied to a solenoid coil to move the plunger against the resilient force of a spring. Therefore, if the pressure of an outlet port or an exhaust port is higher than that of an inlet port, the valve doesn't block a backward flow. Further, while a solenoid valve for anti-lock brake system requires a response time of 8 ms or below, the response time of the conventional solenoid valve becomes long for the reason that the movement of the ball is caused by the pressure difference to operate the valve. Moreover, it is impossible that the response time is kept constant, because of fluctuations of the pressure difference of fluid.